The Kahramanmaraş
Earthquake Doublet
6 February 2023  ·  Turkey & Syria

Executive Summary

The Kahramanmaraş earthquake doublet of 6 February 2023 is the most important recent case study in the catastrophic consequences of the gap between building code design and building code enforcement. Turkey possessed, at the time of the earthquake, a modern, detailed, and internationally benchmarked seismic building code, one that, had it been consistently applied and enforced, would have substantially reduced the death toll and building collapse rate. A new independent field investigation found that despite established technical know-how, state-of-the-art building codes, and rigorous building regulations, a drive for profit pushed all players within the construction industry to take shortcuts, with building stock primarily made of reinforced concrete structures being the main cause of casualties.

The seismology was extraordinary, a doublet of two Mw 7.7+ earthquakes within nine hours on an interacting fault network, producing more than 500 km of combined surface rupture and rupture dynamics that surprised seismologists. The sequence ruptured at least six faults, following unexpected rupture trajectories that included delayed backward branching, statically and dynamically aided triggering, and a combination of subshear and supershear rupture episodes. But the geophysical complexity of the doublet, while scientifically remarkable, explains only a fraction of the death toll. The dominant driver of fatalities was the systematic failure of multi-storey reinforced concrete buildings that should, under a properly enforced code, have survived.

The earthquake sequence became the deadliest global disaster since 2010, and by far the costliest and deadliest in Turkey's modern history. The earthquakes in Turkey and Syria were responsible for nearly half of the total global economic losses in the first half of 2023, estimated at USD 91 billion. Against insured losses of approximately USD 5–6 billion, the protection gap exceeded 93%, an extraordinary figure for a country that has operated a compulsory residential earthquake insurance pool since 2000.

What Happened - The Earthquake Doublet

The East Anatolian Fault - A Known but Underestimated Hazard

The East Anatolian Fault (EAF) is one of the world's major active transform faults, a left-lateral strike-slip system accommodating the westward escape of the Anatolian microplate as it is squeezed between the converging Eurasian and Arabian plates. The EAF extends approximately 550 km, accommodating 6–10 mm per year of relative plate motion. The fault was well-known and well-mapped, its seismic hazard documented in Turkey's national probabilistic seismic hazard assessment. The question was not whether the EAF could generate a large earthquake, it was when. Based on the fault's slip rate and the strain accumulated since its last major rupture, the answer pointed clearly to a significant seismic deficit. The unexpectedly-large slip on some EAF segments suggests a supercycle lasting ≥900 years, meaning the February 2023 rupture may have released strain accumulated over nearly a millennium.

The Rupture Mechanics - Scientific Surprises

The Mw 7.8 mainshock initiated not on the main East Anatolian Fault itself but on a smaller splay fault, the Narlı Fault, before propagating onto the main EAF and rupturing bilaterally northeast and southwest. This initiation on a secondary fault before cascading to the main system was unexpected and highlights the complexity of rupture dynamics in networks of interacting faults.

The Mw 7.7 aftershock, occurring nine hours later on the separate Çardak-Sürgü Fault approximately 90 km to the northwest, demonstrated bilateral supershear rupture at approximately 5.0–6.0 km/s over an 80 km length. Supershear rupture, where the rupture front propagates faster than the shear wave velocity of the surrounding rock, produces a distinctive and particularly damaging pattern of ground motion, concentrating seismic energy in a Mach cone ahead of the rupture front rather than radiating it omnidirectionally. Its occurrence on the Mw 7.7 contributed to severe near-fault ground motions that exceeded model predictions in several affected cities.

Near-Fault Ground Motion Exceedance

Near-fault damage was significantly underestimated by seismic hazard maps: observed ground motion intensities significantly exceeded predictions at distances less than 10 km from the fault. This exceedance reflected genuine deficiencies in the empirical ground motion models used to construct Turkey's seismic hazard maps, which were not well-calibrated for the extreme near-fault environment of a major strike-slip rupture. The supershear Mach wave effects of the Mw 7.7 compound this further, empirical GMPEs are calibrated primarily to subshear ruptures, and do not accurately capture the directionally concentrated radiation pattern of supershear propagation.

Why the Losses Were What They Were

The Building Failure Catastrophe - Code Existence vs. Code Compliance

The death toll of 53,000 people from a pair of Mw 7.7–7.8 earthquakes in a country with a modern seismic building code demands an explanation. The answer is not that the earthquakes were unexpectedly powerful, ground motions in most affected cities, while severe, were within ranges that well-engineered buildings should have survived. A large proportion of the building stock, particularly the multi-storey reinforced concrete residential buildings that housed most of the victims, was not well-engineered.

Turkey has progressively updated its seismic building code since the catastrophic 1999 Marmara earthquakes. The current Turkish Building Earthquake Code (TBEC-2018) is a modern, internationally benchmarked standard that, if consistently applied, would produce buildings capable of surviving major earthquake shaking with limited life-safety risk. The problem was that compliance was neither universal nor effectively enforced:

• Illegal construction (kaçak yapı): A significant proportion of Turkey's housing stock was built without permits, without engineering oversight, and without inspection, particularly in rapidly urbanising cities in the affected region
• Construction amnesty programmes: Government programmes allowed builders and owners to pay fines instead of adhering to codes for years leading up to the disaster. These practices allowed responsible parties to effectively pay their way out of creating safe and resilient infrastructure, retroactively legalising illegal buildings and removing the financial incentive for code compliance
• Material substitution: Field investigations found widespread use of substandard concrete, insufficient cement content, poor water-cement ratio, or inappropriate aggregates. Laboratory testing found compressive strengths substantially below code minimums
• Rebar deficiency: Longitudinal and transverse reinforcement was insufficient in many collapsed buildings, in quantity, quality, or detailing at critical column-beam joints
• Soft storey configurations: Many collapsed buildings exhibited soft storey failure, where the ground floor is weaker than upper floors (typically open for commercial use or parking), causing collapse at ground floor level while upper storeys pancake downward. One of the most well-understood and easily preventable failure modes in earthquake engineering, yet it dominated the collapse pattern

The 1999 Marmara Parallel - A Warning Ignored

Kahramanmaraş was not Turkey's first experience of catastrophic building failure in a major earthquake. The 1999 Mw 7.6 Kocaeli earthquake killed 17,000 people through nearly identical failure modes, soft storey collapses, pancake failures, substandard concrete. That event produced the same engineering analyses, the same condemnations of the construction industry, and the same political commitments to reform that followed February 2023. The TCIP compulsory insurance system was created directly in response to 1999. The building code was substantially updated.

Twenty-four years later, in a different part of Turkey but with the same construction practices, the same failure modes killed three times as many people. The persistence of the same vulnerability through two major seismic events and multiple reform commitments illustrates a fundamental challenge in earthquake risk management: the governance and political economy of building code enforcement are often more determinative of outcomes than the technical content of the code itself.

The Syria Dimension

The earthquake's impacts extended across the border into northern Syria, a country already devastated by more than a decade of civil conflict. The population in the affected area was among the most vulnerable in the world: internally displaced persons and communities whose infrastructure had already been severely degraded by years of conflict. Post-earthquake emergency response was severely hampered by damaged infrastructure, absence of functioning government systems, ongoing security constraints, and the near-total absence of insurance and financial recovery mechanisms available in Turkey. Syria suffered substantial physical destruction and a significant share of the fatalities, and generated almost no insured losses, because the country essentially had no functioning property insurance market in the affected region.

The Insurance Architecture - A System That Fell Short

Turkey established the Turkish Catastrophe Insurance Pool (TCIP/DASK) following the 1999 Marmara earthquake, a pioneering compulsory residential earthquake insurance system, heavily reinsured in the international market with Munich Re and Swiss Re among leading capacity providers. In principle, TCIP represented exactly the kind of broad-based mandatory system that disaster risk finance experts advocate. In practice, its performance revealed several structural limitations:

• Penetration gap: Despite compulsory cover, insurance penetration in the affected area hovered around 50%. Compulsory in principle, the system relies on proof of insurance at the point of mortgage registration, informal constructions in poorer areas often lack formal registration, creating a structural gap precisely where vulnerability is highest
• Underinsurance within TCIP: Policyholders underestimated floor areas to reduce premiums, introducing a protection gap for those considered to be insured
• Commercial lines excluded: TCIP covers residential buildings only, commercial property, industrial facilities, and public infrastructure penetration in the affected southeastern provinces was lower than the national average
• Syria excluded: TCIP is a Turkish national system. Syrian losses had no equivalent public insurance backstop

The Insurance Association of Turkey estimated total losses to the private insurance sector at approximately USD 4.0 billion. The TCIP received nearly 600,000 claims, with total payments expected to reach approximately USD 1.6 billion. Combined insured losses of approximately USD 5–6 billion against economic losses exceeding USD 91 billion represents a protection gap of approximately 93–94%, among the largest recorded for a major earthquake in a middle-income country with a functioning insurance market.

What the Cat Models Got Wrong

Hazard Model Performance

The East Anatolian Fault was a known hazard source in all commercial cat models for Turkey, this was not a blind fault surprise in the manner of Canterbury. The modelling failures were more subtle:

• Near-fault ground motion exceedance: Observed intensities significantly exceeded predictions at distances less than 10 km from the fault. GMPEs had not been adequately calibrated to the near-fault environment of a major EAF rupture
• Multi-segment cascading rupture: The simultaneous failure of multiple fault segments, and the stress-triggered Mw 7.7 on a separate fault nine hours later, represents a fault interaction complexity that pre-event probabilistic models had not fully incorporated
• Supershear effects: The supershear rupture of the Mw 7.7, with its distinctive Mach wave radiation pattern, is not captured in standard empirical GMPEs, areas in the supershear rupture direction experienced systematically higher ground motion than models predicted

Vulnerability Model Performance

The most significant cat model failure was in the vulnerability domain. Vulnerability functions for Turkish reinforced concrete construction, calibrated to the performance of buildings assumed to comply with the building code, substantially underestimated the actual damage rates observed. Despite relatively strict and modern building codes currently in place in Turkey, structural integrity and performance varied widely in the affected regions, with many total collapses of newly built multi-storey residential buildings.

Vulnerability functions calibrated to code-compliant construction will systematically overestimate structural performance in environments where compliance is low and variable. The gap between what a building is called, reinforced concrete frame, 5-storey residential, and what it actually is in terms of structural quality was the dominant source of cat model underestimation in Kahramanmaraş.

Chronological Record

Kahramanmaraş vs. Canterbury - Building Code and Enforcement

Factor	Kahramanmaraş 2023 (Turkey)	Canterbury 2010–2011 (New Zealand)
Earthquake magnitude	Mw 7.8 + Mw 7.7 doublet on known EAF	Mw 7.1 + Mw 6.2 sequence on blind faults
Primary cat model failure	Vulnerability functions not calibrated to non-compliant construction; near-fault GMPE exceedance	Blind fault source absent from hazard model; liquefaction not modelled
Dominant damage mechanism	Building collapse from structural non-compliance	Liquefaction and lateral spreading (suburban); building collapse (CBD)
Building code status	Modern, internationally benchmarked code existed, widely circumvented	Modern code, generally complied with; pre-code stock vulnerable
Fatality driver	Pancake and soft-storey collapse of occupied multi-storey RC buildings	CTV building collapse (115 deaths); widespread liquefaction damage
Protection gap	~93–94%, TCIP covers ~52% of eligible structures; underinsurance within covered	~23%, EQC system with high residential penetration
Key modelling legacy	Non-compliance vulnerability adjustment; near-fault supershear GMPE revision; multi-fault cascading rupture	Blind fault inclusion; liquefaction explicit modelling; aftershock sequence frameworks

The Istanbul Warning

The Kahramanmaraş sequence focused global attention on the unresolved seismic risk in Istanbul, a city of 16 million people built largely on similarly non-compliant reinforced concrete construction, sitting above the North Anatolian Fault which has been seismically loaded since the 1999 Marmara earthquake. With only 20% national TCIP penetration and approximately one-third of all policies in Istanbul, the system exhibits signs of adverse selection, inadequate premium structure, and insufficient funding. The annualised earthquake losses in Istanbul are estimated at USD 140–300 million. Even with a raised deductible, earthquake losses following a large Istanbul earthquake will approximately reach the current capacity of the TCIP, or exceed it.

The structural vulnerability exposed in southeastern Turkey, soft storey buildings, substandard concrete, illegal construction amnesties, is replicated at vast scale in Istanbul, where millions of buildings were constructed before the 1999 code reforms, many on soft alluvial and landfill sites with high liquefaction potential. The probability of a large earthquake on the North Anatolian Fault beneath Istanbul is estimated at approximately 50–70% over the next 30 years. Kahramanmaraş demonstrated in precise and devastating terms what that probability implies in human and financial terms if building vulnerability is not addressed before the next event arrives.

Legacy - What Kahramanmaraş Changed

Summary - Key Analytical Takeaways

1. Building code existence and compliance are not the same variable: Turkey had a modern, benchmarked seismic building code. The code did not protect the people who died, because the buildings they lived in did not comply with it. Cat models that assign vulnerability based on construction type and design era, without adjusting for jurisdiction-specific compliance rates, systematically underestimate actual damage in low-enforcement environments.
2. Construction amnesty programmes are a cat modelling risk factor: Government programmes that retroactively legitimise non-compliant construction remove the economic incentive for code compliance, eliminate the legal basis for enforcement action, and directly embed structural vulnerability into the urban fabric at large scale, with measurable consequences for earthquake loss distributions.
3. Compulsory insurance requires compulsory enforcement: TCIP is a well-designed system in principle, undercut by informal construction outside the registration system, premium optimisation through floor area underreporting, and lower penetration rates in the most hazard-exposed and least prosperous regions, precisely where coverage is most needed.
4. Multi-fault cascading rupture requires multi-fault stochastic catalogs: The nine-hour doublet, Mw 7.8 on the EAF followed by Mw 7.7 on the separately-triggered Çardak-Sürgü Fault, represents a scenario that single-fault rupture stochastic catalogs cannot capture. Complex fault networks can generate correlated multi-rupture sequences whose combined loss substantially exceeds the sum of individually-modelled single-fault scenarios.
5. The Istanbul scenario demands urgent attention: Every structural, regulatory, and insurance deficiency exposed by Kahramanmaraş is present at greater scale in Istanbul. The North Anatolian Fault beneath the Sea of Marmara has a well-characterised probability of generating a major earthquake within the next three decades. If the lessons of February 2023 are not translated into enforced building code compliance and genuine insurance penetration before that event occurs, the consequences for a city of 16 million people will be on a different order of magnitude entirely.